Detonation, or combustion knock, has been particularly problematic in reciprocating spark ignited internal combustion engines for many years. In fact, detonation is one of the most studied aspects of the internal combustion engine. In reciprocating spark ignited engines, for example, after spark ignition, a flame travels outward from the spark plug and will progressively burn the entire fuel-air charge in the combustion chamber (cylinder) under normal combustion conditions. The burned gas generates heat and expands, leading to increased temperature and pressure in the unburned gas (end gas) ahead of the flame front. Additionally, cylinder pressure and temperature are increasing due to the upward, compressing motion of the piston in its compression stroke. This may cause the unburned gas to be raised above its autoignition temperature, wherein the unburned gas will spontaneously ignite and burn nearly instantaneously. Thus, if the flame front velocity in the cylinder is low or if the chemical reactions which precede autoignition occur too quickly, the unburned gas will autoignite. This spontaneous combustion results in a very intense pressure wave that is internally reflected within the cylinder wall at a characteristic frequency related to the velocity of sound within the cylinder and the dimensions of the combustion chamber. The flexing of the cylinder wall and cylinder head resulting from this intense pressure wave may produce an audible high-frequency pinging sound known as combustion knock or detonation. The particular fundamental detonation frequency is controlled primarily by the combustion chamber temperature and the cylinder bore diameter. Thus, different engine models or sizes experience detonation at varying characteristic frequencies.
In reciprocating spark ignited internal combustion engines, it is desirable to maintain the spark advance close to the onset of detonation in order to improve fuel efficiency. However, detonation is highly objectionable because of the damage which it causes. For example, in addition to the unwanted sound, detonation can result in erosion of the combustion chamber, damage to spark plug electrodes, exhaust valve damage, or piston ring damage. Thermal damage from detonation is most likely to affect lower melting temperature materials such as aluminum pistons, or parts with poor heat transfer paths to a heat sink such as exhaust valves. Additionally, detonation can induce preignition (i.e., ignition before the spark occurs) which leads to loss of engine efficiency, and rough and unsatisfactory operation. Typically, engines without detonation sensing devices operate with the ignition timing retarded sufficiently to avoid detonation under all expected conditions. This retarded ignition timing results in reduced fuel efficiency. Also, as just explained, engine damage may occur if some unexpected condition causes detonation in an engine with no detonation sensing capability. Accordingly, it is highly desirable to minimize the number and severity of detonations that an engine cylinder is exposed to.
Various factors contribute to the existence of detonation such as engine design parameters and engine operating variables. For example, engine designs that increase the temperature, pressure, and chemical residence time of the end gas increase detonation. Other engine design factors that increase detonation include increased compression ratio, off-center spark plug location, and slow-burn combustion chambers. Additionally, engine operating variables such as spark advance, revolutions per minute, throttle angle, coolant temperature, intake air temperature and humidity, and air/fuel ratio can all have a significant influence on detonation.
Increasingly, use has been made of electronic engine controls with detonation sensors to control the occurrence of detonation. The two most common methods of sensing detonation are monitoring the pressure waves created by detonation, or detecting the resulting vibrations of the cylinder walls due to detonation. Various types of instruments have been developed for detecting pressure waves in the cylinders created by detonation. For example, a pressure transducer or diaphragm can be located in a plug or similar device which is screwed into the cylinder, with the transducer generating an electrical signal as a result of motion of the transducer from pressure waves. In order to provide the electrical signal, the transducer can include a strain-gauge element, a piezoelectric crystal or a magnetic circuit, for example. The electrical signals from the transducer corresponding to the detonation pressure are then transmitted to an engine controller, which takes appropriate action to reduce or eliminate detonation. A significant disadvantage of pressure-type detonation sensors is their location with respect to the engine. The extreme temperatures generated by the engine and the intense pressure fluctuations which these sensors are subjected to significantly affect their reliability. As a result, the use of pressure-type sensors has been very limited.
Currently, the most prevalent type of detonation sensors, such as accelerometers and strain gauges, detect vibrations of the cylinder walls induced by detonation. Accelerometers are sensitive to detonation vibrations and translate these vibrations into electrical signals. Many types of accelerometers exist, and one type often used to detect detonation utilizes a piezoelectric crystal to transform mechanical vibrations into electrical signals. Accelerometers also differ with respect to their resonant frequencies. For instance, spike resonant and broadband resonant accelerometers generally center on the mean detonation signal frequency of the engine, with spike resonant sensors having narrow bandwidths on the order of 100 Hz, and broadband resonant sensors having bandwidths approaching 1000 Hz. These sensors thus utilize their built-in mechanical amplification and filtering characteristics to detect particular detonation signal frequencies. However, because of the relatively narrow bandwidths of these sensors, one sensor is not capable of operating with various engine models and sizes. On the other hand, flat response accelerometers, which have a high resonant frequency and a much wider bandwidth than spike resonant and broadband resonant accelerometers, offer the advantage of commonality since one sensor design can be utilized to detect different detonation signal frequencies for several engine models. In order to distinguish between detonation signals and noise of the engine, however, flat response accelerometers must operate in conjunction with electrical filtering circuitry.
Known detonation detection systems which utilize accelerometers typically mount the accelerometer on the engine block at thinner and less stiff wall areas to detect detonation induced vibrations. In one prior art system, a magnetostrictive accelerometer is used as the detonation sensor, and is mounted on the intake manifold of the engine. The output of the detonation sensor is transmitted to an electronic engine controller, which includes a detonation filter circuit to condition the signal to differentiate detonation signals from normal engine vibrations or background noise. In order to detect the presence of detonation, the electronic controller continuously monitors the background noise of the engine, and then compares the background noise level with the signal from the detonation sensor to determine if detonation is present. If detonation is detected, the controller then produces a retard command to delay or retard the ignition pulse to the spark plug in an attempt to eliminate detonation.
A significant disadvantage with this type of detonation detection system is that the accelerometer is mounted directly on the engine. Similar to pressure-type detonation detectors but to a somewhat lesser extent, the high temperatures generated by the engine can affect the reliability and durability of the accelerometer. Additionally, prior art detonation detection systems which utilize accelerometers experience problems at high engine speeds where the engine generates greater mechanical vibrations, resulting in reduced signal to noise ratios. Similarly, and of great importance, prior art detonation detectors have not been capable of detecting low intensity, sub-audible detonation, since these detectors do not have the capability of adequately distinguishing between low intensity detonation signals and background noise.